`sysroot` is not documented in `rust-project.json` docs

[abrisco]

It was revealed on bazelbuild/rules_rust#1641 that sysroot is an impactful value in rust-project.json files but this field is not documented on https://rust-analyzer.github.io/manual.html#non-cargo-based-projects

It would be helpful if someone could do a pass and make sure the documentation there is up to date.